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12066.002.000 San Joaquin County Environmental Health Department November 30, 2018 Mountain House Neighborhood A, Units 3 and 4 Revised January 22, 2019 SOIL GAS CHARACTERIZATION REPORT Page 2 SITE CHARACTERIZATION Soil Gas Sampling Soil gas sampling was conducted in two events: November 20, 2018: 10 soil gas samples at five locations along the petroleum pipeline. January 10, 2019: 3 soil gas samples at three locations at the west area of the petroleum pipeline, approximately 150 feet southwest of the proposed residential lot areas. November 20, 2018 Sampling Event Ten soil gas samples were collected from temporary nested well points at five locations (Figure 2). Samples were collected from a depth of approximately 5 and 10 feet below the ground surface (bgs). The soil gas samples were collected from the temporary probes advanced with a Geoprobe® direct push rig and constructed in general accordance with the Department of Toxic Substances Control (DTSC) Final Advisory Active Soil Gas Investigations (April 2012) and under permit from the San Joaquin County Environmental Health Department. Figure 3 provides the typical nested soil gas well detail. The temporary soil gas wells were constructed with 1h-inch diameter Teflon® tubing equipped with a filter at the base of the tubing. The temporary well installation was performed with a direct push probe rig, which advanced an approximately 3-inch-diameter boring to a depth of 10.5 feet bgs. For each location, the bottom of the well casing was equipped with a filter situated at a depth of 10 feet below the ground surface, centered in the middle of a 1-foot layer of sand. Four feet of hydrated bentonite grout was installed on top of the sand. Then a second filter was placed in the middle of a 1-foot layer of sand and the remaining annular space was filled to the surface with hydrated bentonite grout. Once the installation of the annular seal was complete, the well casings were equipped with a permanent Swagelok® ferrule and nut. The wells were left, allowing for the mandatory two-hour equilibration time to commence. After the two-hour equilibration time elapsed, the sample train was connected to the well tubing by threading the permanent Swagelok® fitting on the well casing onto the manifold. The sample train consisted of a stainless steel twin summa quick-connect manifold with built in flow controller set to 80-180 ml/min. A purge vacuum canister was attached to the manifold connection closest to the well casing and the sample canister was connected to the manifold fitting furthest away from the well casing. An appropriate volume, approximately two liters of soil gas was then purged from each well and the purge valve was closed. Prior to sampling, a leak test was performed to establish if there was a leak during sampling. The leak test consisted of using aerosol 1,1-difluoroethane on a rag placed over the manifold on the sample canister. Soil gas samples were collected by opening the valve on the summa canister provided by the laboratory, while the purge valve is closed. Following sampling, each sample canister was labeled with a unique identification number, sampling time, pre- and post-sample vacuum readings. The sample canisters were submitted under documented chain of custody to Torrent Laboratory, Inc. A State-accredited laboratory in Milpitas, California for analysis.